Fiber optic gyroscopes based on the "ring" interferometer configuration currently rely on the measurement of the Sagnac phase shift induced between clockwise (cw) and counterclockwise (ccw) propagating optical beams developed from a single light signal. The magnitude of the Sagnac phase shift is given by ##EQU1## where A is the cross-sectional area of the fiber coil, N is the number of fiber turns, .lambda. is the wavelength of the light, and .OMEGA. is the rotation rate. Generally, high sensitivity measurements require the detection of phase shifts smaller than 10.sup.-6 radians. Due to the interferometer transfer function, however, the output becomes nonlinear at higher rotation rates, and if 2.phi..sub.S exceeds .pi./2 radians the output becomes ambiguous.
Several schemes have been suggested as means for providing an extended dynamic range. Two such schemes are described in an article entitled "Two Wider Dynamic-Range Signal-Recovery Schemes for the FiberOptic Gyroscope," A. C. Lewin et al, IEE Proceeding, Vol. 132, No. 5, October 1985. Typically such schemes rely on the linearization of the interferometric transfer function over each 2.pi. interval, use one carrier frequency, or adopt fringe counting electronics to track the total phase excursion. Although this mode of operation is desirable, the ambiguity associated with the periodic interferometer output is not completely eliminated. For instance, if the device is powered up in a rotating frame, the output is ambiguous by a factor 2m.pi., where m represents any integer. This limitation is common to most types of demodulation schemes used with a gyroscope, but is not generally considered a serious disadvantage as the operational limits often mean the device is operated within the zero order fringe. Higher dynamic range systems which would be required to operate beyond the zero fringe would require that each fringe be counted and would therefore be limited by this effect.
The current methods make the development of sensors for D.C. measurables, i.e., temperature or pressure where fringe counting is normally required, impossible.
An additional problem of the methods is the inability to provide fringe numbers which are accurate enough to enable high resolution output for high sensitivity measurements to be provided.